1. Field of the Invention
This invention relates to swapping circuits in a metal-only, engineering change only (ECO) process using wire swaps to allow for better slews and timing.
2. Description of Background
This invention relates to an engineering change only (ECO) process where timing and slews can be improved by effectively swapping similar circuits using wiring changes. Typically, in IBM's engineering processes wire-only changes are used to make minor tweaks to a chip design by keeping all the silicon masking while changing the wire masks in the chip fabrication process.
In fact, even logic circuits can be created with metal, provided special ‘filler cells’ are used. These filler cells contain un-customized transistors. By connecting up these transistors in the right way, different logical functions can be created (i.e. NAND3, OR2, Inverter, etc.).
This way, wafers can be created in large quantities up through the wire customization steps. Then, if new problems are detected in the design, the design fixes can be made simply using wire changes that get applied to make new masks to apply to the existing sub-stock. The effect is a quicker chip fabrication from the last detected problem/fix until chip availability.
One problem with metal-only engineering changes is that the metal-customized filler cell gates are generally not as fast as original silicon gates due to the limitation of making the filler cells growable. Also, it is difficult to provide all the flavors of filler-cell books (due to high cost of design and modeling efforts) and there is a likelihood that some of these books would never get used. There may also not be enough contiguous space for some of the larger books required in an engineering change since much of the space may have already been used up.
It would be desirable to have a way to design using filler cells that were just as good as original silicon books due to the cost of modeling efforts. Also, if a new path becomes the most critical path in an area, it would be nice to have the flexibility of re-choosing which circuits get used for which applications to achieve the same function with better performance.